One error control technology is called automatic repeat request (hereinafter “ARQ”). In ARQ, a transmitting side and a receiving side are linked by a bi-directional channel. Initially, the transmitting side sends packets that include code words generated by applying error detection encoding to information bits, and the receiving side applies error detection. If the receiving side does not detect an error in the received data, the receiving side returns a reception acknowledgement signal (called positive acknowledgment or “ACK”; hereinafter referred to as “ACK”) to the transmitting side to indicate that the reception is correctly performed. If the receiving side detects an error in the received data, the receiving side returns a retransmission request signal (called negative acknowledgment or “NACK”; hereinafter referred to as “NACK”) to the transmitting side. The transmitting side retransmits the same packets when the NACK signal is received. The transmitting side repeats retransmitting the same packets until the ACK signal received. ARQ makes it possible to implement high-quality data transmission.
If a handover is started in midstream of error control by the ARQ, the packets to be resent are destroyed at the handover source and the same packets are transmitted at the handover destination, so there is the problem of reduced throughput.
In view of the above problem, patent document 1 proposes a communication system that prevents a drop in throughput by discarding and retransmitting retransmission packets when there is a handover by reducing the number of retransmission packets, by limiting the number of processes for retransmission if there is the possibility of a handover, considering the possibility of a handover.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-80640